Heat transfer enhancement by louvering or slitting plate fin surfaces in heat exchangers has long been recognized as offering significant improvements in plate finned coil performance. The form and arrangement of the louvers are unique to the type of plate fin surface used in the particular heat exchanger since the airflow characteristics vary with the type of plate fin surface. The airflow characteristics of a surface depend upon whether the surface is flat, corrugated or wavy, and depend upon the arrangement of the heat transfer tubes. Most surfaces known today increase the heat transfer performance of the coil when the heat transfer surface is dry, such as when the coil is used as a refrigerant condenser. However, when the surfaces are wet, such as when the coil is used as an evaporator, the heat transfer performance is not improved by louvering or slitting the plate fin surface. Additionally, many previous plate fin surfaces suffer from high airside pressure drop, which means that more power is required to move air through the coil.
U.S. Pat. No. 4,860,822 discloses sinusoidal plate fin surfaces having lances located at each peak and trough in the area between the heat transfer tubes. Similarly, European patent application EP 0 325 553 Al discloses sinusoidal plate fin surfaces having apertures located at each peak and trough in the area between the heat transfer tubes. U.S. Pat. Nos. 4,817,709 and 4,787,442 clearly show "delta wings" and "ramps" located after each peak and trough in the area between the heat transfer tubes. U.S. Pat. Nos. 4,614,230 and 3,397,741 are examples of patents which show a slight gap between the heat transfer tubes but still disclose louvers located in the areas between the heat transfer tubes. Neither of these last mentioned patents are directed to wavy plate fin surfaces, which means that their airflow characteristics will vary considerably from the airflow characteristics of a wavy plate fin surface.